A single flux quantum (hereinafter, abbreviated as SFQ) circuit having a single flux quantum (Φ0=h/2e=2.07×10−15 Weber) as an information carrier is a superconducting circuit featuring an ultrafast operation above tens of GHz (109 Hz) and a low power consumption characteristic below microwatt (μW) per gate. Based on the principle shown in Non-Patent Document 1 (IEEE Trans. on Appl. Supercond., vol. 1, No. 1 (1991) p. 1), various logic gates have been developed, and the development of practical circuits combining these with each other is widely advanced.
As an application of an SFQ circuit, there is an analog-to-digital (hereinafter, abbreviated as A/D) converter. To exploit the feature of the SFQ circuit, an oversampling method-is employed for the conversion method. In this method, an input signal is sampled (oversampled) at a frequency sufficiently higher than a bandwidth with accuracy of a small number of bits, and its sampling data signal is then signal-processed to obtain a digital data signal with high accuracy in a necessary bandwidth. A front-end circuit performing oversampling is constructed of the SFQ circuit operable at tens of GHz, and a back-end circuit performing signal processing is constructed of a semiconductor circuit excellent in integration properties, thereby realizing a high-performance A/D converter.
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2002-374152
[Patent Document 2]
Japanese Patent Application Laid-Open No. 2001-345488
[Non-Patent Document 1] IEEE Trans. on Appl. Supercond., vol. 1, No. 1 (1991) p. 3